I. Field of the Invention
The present invention relates to variable gain amplifiers (VGAs) and particularly to VGAs used in communication devices.
II. Description of the Related Art
In a wireless communication environment, a wireless communications receiver may receive a signal which experiences rapid and wide variations in signal power. In receivers such as are used in a wideband digital code division multiple access (CDMA) mobile station, it is necessary to control the power of the demodulated signal for proper signal processing. Furthermore, in transmitters such as are used in a CDMA mobile station, it is necessary to control the transmit power in order to avoid excessive interference to other mobile stations. These same power control considerations apply to narrowband analog frequency modulation (FM) wireless communication system receivers and transmitters.
Dual-mode CDMA/FM wireless communications devices exist which are required to provide power control of transmitted and received signals of both digital CDMA and analog FM modulation. In these dual-mode mobile stations, the control process is complicated by the differing dynamic ranges and industry regulation standards associated with the CDMA and FM signals. That is, the magnitude of the received CDMA signals may vary over a range of approximately 80 dB, whereas the magnitude of the received FM signals may vary over a range of as much as 100 dB. The provision of separate automatic gain control (AGC) circuitry for both the CDMA and the FM signals increases the complexity and expense of such dual-mode mobile stations. Accordingly, it is desirable to provide AGC circuitry capable of operating upon both the CDMA and FM signals.
FIGS. 1A and 1B illustrate an exemplary environment for a VGA performing AGC functions. FIGS. 1A and 1B are a block diagram of a dual-mode CDMA/FM cellular telephone 900 designed, for example, in accordance with the telecommunication industry standard "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System," TIA/EIA/IS-95, generally referred to simply as IS-95. A VGA is used for receive and transmit AGC amplifiers 902, 904 respectively, of cellular telephone 900. The front end receiver portion of cellular telephone 900 comprises antenna 906, duplexer 908, low noise amplifier (LNA) and mixer circuit 910, and filter 930. As cellular telephone 900 travels throughout the coverage area of a CDMA system, the signal level at antenna 906 varies from about -110 dBm to -30 dBm. Note that each of these front end elements generally provides the same gain no matter what signal level is applied to it over the operating range such that the dynamic range of the signal which is applied to receive AGC amplifier 902 is the same as the dynamic range of the signal at antenna 906, approximately 80 dB. Similarly, when the cellular telephone 900 travels throughout the coverage area of an FM system, the signal level at the antenna varies approximately 100 dB.
The output of receive AGC amplifier 902 is provided to baseband analog application specific integrated circuit (BAASIC) 912 which converts the analog signal to a digital signal. The analog to digital signal conversion process works best if the signal level which is applied to the analog to digital converter remains constant. Receive AGC amplifier 902 performs the function of compensating for the variations input power such that the output power of receive AGC amplifier 902, and thus the input to the analog to digital converter, remains constant.
Mobile station modem ASIC 914 provides demodulation for both the CDMA and FM signals, as well as various digital and power control functions associated with CDMA operation. Such functions are well known in the art and not critical to the present invention, and thus are not described further herein. User interfaces 916 provide the interface to the human operator. Such user interfaces 916 are also well known in the art and not critical to the present invention, and are thus not described further herein.
Mobile station modem ASIC 914 also provides a baseband modulated digital representation of the CDMA waveform, or a modulated analog representation of the FM waveform to BAASIC 912. BAASIC 912 converts the baseband signals representation to analog intermediate frequency (IF) form at a constant signal level and supplies it to transmit AGC amplifier 904. Transmitter AGC amplifier 904 provides power control to the signal and supplies it to upconverter 918, power amplifier and driver circuitry 920, isolator 922, duplexer 908 and antenna 906. As cellular telephone 900 travels throughout the coverage area of a cellular system, the transmit signal level at antenna 906 varies inversely from receive power in that when the receive power is at a minimum the transmit level is near the maximum. This variation in transmit power level is accomplished by AGC amplifier 904. Note that the input power to AGC amplifier 904 is typically fixed, and the gain of power amplifier 920 may also be fixed.
More information about the automatic gain control loop in a wireless communication system and about power control in general can be found in U.S. Pat. No. 5,283,536, entitled "HIGH DYNAMIC RANGE CLOSED LOOP AUTOMATIC GAIN CONTROL CIRCUIT" issued Feb. 1, 1994, U.S. Pat. No. 5,107,225, entitled "HIGH DYNAMIC RANGE CLOSED LOOP AUTOMATIC GAIN CONTROL CIRCUIT" issued Apr. 21, 1992, U.S. Pat. No. 5,267,262 entitled "TRANSMITTER POWER CONTROL SYSTEM" issued Nov. 30, 1993, U.S. Pat. No. 5,469,115 entitled "METHOD AND APPARATUS FOR AUTOMATIC GAIN CONTROL IN A DIGITAL RECEIVER" issued Nov. 12, 1995 and U.S. Pat. No. 5,283,536 entitled "HIGH DYNAMIC RANGE CLOSED LOOP AUTOMATIC GAIN CONTROL CIRCUIT" issued Oct. 26, 1993, each of which is assigned to the assigned hereof and incorporated herein by reference.
Mobile communication receivers and transmitters like those described above are designed to have a high compression point, low noise injection and low power consumption. Receivers with a high compression point and low noise injection have a high dynamic range in that they can detect signals over a wide range of power levels. Transmitters with a high compression point and low noise injection have a high dynamic range in that they can transmit signals over a wide range of power levels. Receivers and transmitters with low power consumption increase battery life. Hence, these characteristics are important when designing a variable gain amplifier for a communication system in which signals are transmitted and received over a large range of power levels.
A receiver should be able to detect information from both a strong signal broadcast by a nearby and powerful transmitter and a weak signal broadcast by a distant and low power transmitter. The extent over which the receiver can detect weak to strong signals is termed its dynamic range. Likewise, a transmitter should be able to transmit low powered signals to a nearby receiver and high power signals to a distant receiver.
The dynamic range of a receiver is established by its minimum detectable and maximum detectable signal levels. The minimum detectable signal level of a receiver is determined by the receiver's noise figure. Likewise the minimum transmittable power is set by the transmitters noise figure if the signal level falls near or below the noise floor. A VGA's noise figure is in part a function of the noise injection properties and gain of the VGA. In general, the higher the receiver's gain, the better it's noise figure; i.e. the better able it is to detect a very weak signal in the presence of noise.
The maximum detectable signal level of a receiver may be established by the receiver's intermodulation distortion (IMD) performance. When multiple signals pass through any device, mixing action between the signals occurs because of the non-linearities of the device. For example, in a location where CDMA and analog FM systems co-exist, third-order IM products from the analog FM system generally fall within the CDMA passband. This IM products act as "jammers" that contribute to IMD which can interfere with detection and demodulation of the desired signal within the receiver. A VGA's IMD performance is in part a function of its linearity and its gain. In general, the lower the receiver's gain, the better it's IMD performance. This is in contrast to the noise figure requirements as described above. Thus, design of a VGA for a receiver with a large dynamic range includes the difficult tradeoff between IMD performance and noise figure.
Similar design considerations are relevant with respect to transmitter VGAs, with the difference being that generally, receiver VGAs are designed to provide a relatively constant output power level for a varying range of input power levels while transmitter VGAs are designed to receive relatively constant input power levels and provide a varying range of output power levels.
Furthermore, mobile receivers are designed to be compact, lightweight, and have a long operating lifetime. Mobile receivers are powered by a minimal number of battery cells to reduce their size and weight to enhance their portability. Because battery voltage is proportional to the number of battery cells, the AGC circuitry, including the variable gain amplifier (VGA), must operate at low supply voltages. It is also desirable to enhance battery lifetime in order to increase the period between battery replacement or recharging. Therefore, the AGC circuitry, including its VGA, should consume little DC current and power.
This requirement for low DC power consumption also implies a design tradeoff similar to that already mentioned. More DC power is required for a high gain amplifier that has good noise figure. However, less DC power is required for a low gain amplifier that has good IMD performance. Existing VGA designs are inefficient in that they are unable to conserve DC power sufficiently at low gain levels.
What is needed is a VGA with a high dynamic range, good noise figure and IMD performance, as well as low DC power consumption.